The dumping of this sludge presents considerable problems. For instance, the readily water-soluble alkalibound phosphates are washed out by rain and this will ultimately result in heavy eutrophication of the local surface water. At the same time, certain portions of the other sludge constituents are also washed out. In a wet phosphoric acid purification unit with an annular throughput of approximately 100,000 tons H.sub.3 PO.sub.4, the quantity of sludge obtained per year will be about 50,000 tons. According to the present state of engineering technology, there is no alternative but to dump this sludge. The cost of such a temporary dump is mainly determined by the land required, plastic film covering and/or lining and by the inspection requirements to ensure that the ground water is not polluted. A final solution to the problem cannot be foreseen at present.
A copending patent application Ser. No. 099,530 now U.S. Pat. No. 4,257,898, Mar. 24, 1981, relates to a method for processing the waste sludge obtained in purification facilities for phosphoric acid produced by the wet process into a P.sub.2 O.sub.5 - bearing fertilizer. The processing method comprises the addition of controlled quantities of the waste sludge to steelwords slag, such that the dry mixture has a P.sub.2 O.sub.5 - content of 3-9%. Further embodiments of this method provide for admixing the waste sludge at a uniform rate to the warm prepared slag or for spreading it fairly uniformly over a hot slag bed.
Whereas the method according to the copending patent application is distinguished by its simplicity, economy and non-polluting properties, it is nevertheless capable of improvement, on the one hand with regard to the high moisture content of the sludge and the resulting difficulties in its handling as well as the tendency to form lumps and incrustation and, on the other hand, with regard to the properties of the end product, while not using any additives, in particular solid or liquid binders, which would render the entire process more costly.